This application explores the homeostatic regulation of APC activity by CD25+ T cells and NK1.1+ cells in a gender and age dependent autoimmune disease resistance/susceptibility model using SJL mice in a Th1 mediated CNS autoimmune disease. Encephalitogenic Th1 cells inducing experimental autoimmune encephalomyelitis (EAE) are activated in females of all ages and mature males (>12 wks old). By contrast, Ag specific Th2 cells are activated in young adult (6 wk old) males, resulting in EAE resistance. Depletion of CD4 + CD25 + T cells from resistant males, but not age matched females, will demonstrate that CD25+T cells suppress Th1 activation. Adoptive transfers will demonstrate that CD25 + T cells from resistant 6 wk males inhibits EAE in female and mature male recipients. Our hypothesis, that the APC alters the CD4 + CD25 + T cells in vivo will be tested by inducing EAE in resistant 6 wk male recipients of APC derived from CD25 + depleted 6 wk male donors. Th1 responses are also induced in EAE resistant males either depleted of NK1.1 + cells or following APC transfer from NK1.1 depleted resistant donors. The hypothesis that the increased frequency of NK cells in resistant males alters the APC via cell-cell interaction is examined by blockade or receptor ligand and analysis of DAP 12 KO mice. Tripartite interactions between CD25 + T cells, NK cells and APC which lead to EAE resistance via Th2 activation are examined by in vivo and in vitro approaches. The influence of each cell type on cytokine secretion, surface and gene expression in the other two cell types will be analyzed following depletion of either the NK or CD25 + T cells. These experiments will provide a mechanistic analysis of cell-cell interactions within a single fixed genotype regulated by both gender and age which ultimately influence resistance or susceptibility to autoimmune disease. These data will also provide the first evidence that NK and CD25 + T cells influence adaptive immunity prior to antigen encounter via alterations in APC function.